Heating garments using resistive wires such as stainless steel, nickel-based alloys or carbonized yarn arranged in a chosen pattern on an electrically insulating backing material as heating elements have found extensive use in heated socks, gloves, jackets, pants, boots, and blankets, as examples. However, such wires are known to have poor flexibility and poor tolerance to frequent bending and contact. Moreover, incompatibility between the expansion properties of the wires and those for the backing material exacerbates these problems.
Sources of electrical energy used to activate such heating garments require controllers to regulate the temperature and to prevent runaway heating thereof. However, in the event that such controllers fail or the resistance of the garment changes rapidly from an electrical short or other situation where the resistance greatly increases, localized heating can cause burns to the wearer.
Phillip Norman Adams et al. in “Conductive Polymer Compositions,”International Publication No. WO 99/24991, which was published on 20 May 1999, teach the synthesis of polyaniline fibers from a solution of polyaniline (˜150,000 g/mol), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) (60 AMPSA molecules per hundred nitrogen atoms in the polyaniline backbone) in dichloroacetic acid. As-spun conductivities for these polymers were found to be between 70±9 S/cm and 90±8 S/cm, when the fiber is spun into butyl acetate and acetone, respectively. Conductivities and tensile strengths were measured to be 810±200 S/cm and 45 MPa, and 1014±200 S/cm and 60 MPa, when the fiber was subsequently stretched to between 5 and 8 times its original length, respectively.
In U.S. Pat. No. 5,422,462 for “Electric Heating Sheet” which issued to Yoshio Kishimoto on Jun. 06, 1995, a unidirectionally conductive electric heating sheet which includes conductive yarns and wires having insulating properties at least on their surfaces that are plain-woven as warps and wefts such that neighboring conductive yarns are not in electrical contact, is described. A list of synthetic polymeric organic fibers currently used in the garment industry is provided as yarn material. These fibers are covered with a conductive layer which includes conductive polymers such as polypyrrole, polythiophene and polyanline, or metals having low melting points. It is stated that the conductive yarn can lose its continuity after breaking its conductive covering layer due to sparks or by overheating. In another embodiment described in the '462 patent, a conductive wire having an insulating layer of thermoplastic polymer on the surface is woven with conductive yarn. When the insulating layer melts as a result of overheating, the conductive yarns and the conducting wires short-circuit and melt, thus functioning as a thermal fuse element. It is clear from this disclosure that the heating sheet cannot be used close to a wearer's skin.
In U.S. Pat. No. 6,074,576 for “Conductive Polymer Material For High Voltage PTC Devices” which issued to Liren Zhao and Prasad S. Khadkikar on Jun. 13, 2000, polymeric positive temperature coefficient (PTC) compositions and electrical devices having a high voltage capability which are capable of operating at alternating current voltages of 110 to 130 volts or greater are described. The PTC compositions disclosed were found to have a high PTC effect of at least 104 to 105 and a low initial resistivity at 25° C. of 100 Ωcm or less. The devices were designed as self-resetting sensors for AC motors to protect these motors from over-heating and/or over-current surges, and can withstand a voltage of 110 to 130 VAC without failure for at least 4 h after reaching the switching temperature, Ts. Such materials include a crystalline or semicrystalline polymer, a particulate conductive filler, an inorganic additive and, optionally, an oxidant. It is known that the Ts of a conductive polymeric composition is generally below the melting point, Tm, which is chosen to be between 100° C. and 200° C. Therefore, once an electrical current sufficient to heat the PTC device is applied thereto, the device retains its electrical and thermal stability after attaining its high electrical resistance at near Tm.
U.S. Pat. No. 6,033,939 for “Method For Providing Electrically Fusible Links In Copper Interconnection” which issued to Birenda N. Agarwala et al. on Mar. 7, 2000 describes methods for fabricating fuses within a semiconductor IC structure, where the fuses are deletable by a laser pulse or by a low-voltage electrical pulse typically below 3.5 V, and are usable to reroute the electrical circuitry of the structure to remove a faulty element. Although the preferred fuse material is silicon-chrome-oxygen and the preferred circuitry is copper, polymers including polyanilines having electrical resistivity in the range between 15 micro-ohm-cm and 90 micro-ohm-cm, are used for the fuse material, since such materials can be spun onto the surface. The heat generated by passing an electric current through the fuse to delete it oxidizes the polyanilines, thereby giving an oxidized material having a very high resistance. The highly resistive, oxidized polyaniline changes color, thereby offering a detector for the changed resistivity. The thin-film fuses are formed using photolithography and etching techniques.
U.S. Pat. No. 5,629,665 for “Conducting-Polymer Bolometer” which issued to James Kaufmann et al. on May 13, 1997 describes an ion-implanted, electrically conductive polymer bolometer fabricated using lithographic techniques. In response to incident infrared radiation, the electrical resistance of the polymer changes. This change can be monitored using a bridge circuit. The polymer film is deposited using spin coating, roller coating or meniscus coating techniques.
It is an object of the present invention to provide conductive-polymer based heating elements suitable for resistive heating applications.
Another object of the present invention is to provide conductive-polymer based resistive heating elements having the light weight, stretchability, flexibility and processability characteristic of commonly used textile fibers.
Yet another object of the present invention is to provide conductive-polymer based resistive heating elements which cannot achieve temperatures sufficiently high to harm a user of a heating apparatus fabricated therefrom.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.